The two most common choices for overrunning clutches in high speed environments, such as shift timing in the gear sets of automatic transmissions, are sprag clutches and roller clutches. Sprag clutches have dumbbell sprags with sloped end surfaces that jam between two confronting cylindrical race surfaces to lock up in one direction, while allowing overrun in the other. In order to work well, the races between which a sprag clutch is installed must be maintained coaxial to one another with a high degree of accuracy, requiring precision bearings. Nor are sprags as strong as rollers, so a large number must be used. However, since they do not spin, sprags are not especially speed sensitive.
A great advantage of roller clutches is that their races need not be maintained coaxial nearly as accurately. The rollers sit in sloped wedging pockets formed between cam ramps on a cam race and the confronting cylindrical surface of a pathway race. The rollers are continually spring biased to a lock up ready position at the narrow end of the wedging pocket, in nearly continual contact with both races. The rollers can travel circumferentially back and forth in the wedging pockets, under their spring bias, remaining at a ready to compensate for the race running eccentricity that continually enlarges and reduces the size of the wedging pockets. The big disadvantage of rollers is that they are more speed sensitive than sprags. Since each roller is in nearly continual contact with both races, traction from the rapidly relatively rotating pathway race can spin it against, and potentially wear on, the cam race. Continually smaller transmissions are operating at faster and faster speeds, to the point where conventional roller clutches may not work. Still, the cost advantage of a roller clutch is so great that a non speed sensitive roller clutch capable of replacing a sprag clutch in high speed applications would be highly desirable.
Recent advances in roller clutch design by the assignee of the subject invention have provided a new approach to controlling roller spin. Rather than directly reducing pathway traction in the first instance, the ability of pathway traction to actually cause the roller to spin is resisted. This is done through the use of a plurality of roller control cars, one for each roller. Each roller is pop fitted very tightly and closely into a respective car, tightly enough that if the roller attempts to spin, the car will turn with it. Side bars on the cars interfitted with side rails of the cage stop the cars from turning with the rollers more than a few degrees, and also keep the cars from contacting the pathway race. After the cars have stopped twisting relative to the cage, pathway traction continues to try to spin the rollers, which then rub on the inside of the tight fitting cars, retarding the spin that would otherwise occur. Roller skew control is also provided, since the rollers are kept square to the cars, and the interfit of the cars with the cage keeps the cars square to the cage. In addition, a releasable latch can be provided to keep the cars locked to the cage during shipping. While such a car is compact, and does not impinge on the space between the races needed for other parts of the cage, there will be clutch environments where pathway traction is so severe, and potential roller spin speeds so high, that a means for directly reducing pathway traction, rather than just retarding the roller spin that it causes, would be desirable.